In hydraulic piston type units, pistons are caused to slide within bores of a rotating cylinder block and act against a swashplate. In so doing, mechanical energy is transformed into fluid power or conversely fluid power can be converted into mechanical energy by reversing the operation. Simply stated, a hydraulic piston type unit of the type described, may be operated as a pump or as a motor. To accomplish this, it is necessary to transfer fluid pressure from one level to another. Where the hydraulic unit is operated as a pump, fluid pressure is raised from a low level to some higher level, whereas, when the hydraulic unit is operated as a motor, fluid pressure is received at a high level and discharged at a lower level. In both of these types of energy transfer situations, it is necessary that a pressure level change or transition take place approximately each 180.degree. of rotation or twice each revolution. It is at these pressure level changes or transitions that slipper-swashplate interface problems occur. A typical problem is that of impact loading of the slipper on the face of the swashplate evidenced by scuffing of the swashplate by the slipper. Another problem is that of erosion typical of that associated with cavitation. The invention to be described hereinafter provides a simple and efficient remedy for these problems.
Historically the problem of cavitation erosion control has been treated in a number of patents to be discussed hereinafter which employ swashplates, hydraulic pumps and motors. A number of these patents recognize the cavitation erosion problem that exists between the main ports of a valve plate and ports in the face of a rotary mounted cylinder block positioned in sliding engagement with the valve plate provided with the main ports. The main ports of the valve plate are generally of a kidney shape and experience either high or low pressure. Movement of the cylinder block containing pistons, alternately exposes the ports in the cylinder block to either high or low pressure.
The patent to Slimm, U.S. Pat. No. 3,369,458, which is directed to a hydraulic apparatus recognizes that there is a sudden pressure change problem between ports 29, FIG. 1 of a rotary cylinder block 25 and fixed high and low pressure ports 33, 34 in a valve plate. An erosion problem that arises because of the sudden pressure changes is relieved by providing an auxiliary port 54, (FIG. 4) in the fixed valve plate 24. The auxiliary port 54 communicates with restricted flow passages 50 and 55 through a check ball valve 58 (FIG. 2 and FIG. 4) in a bridge 52 between the high and low pressure ports. The Slimm hydraulic apparatus has a slipper 41 but makes no provision for slipper-swashplate erosion control.
The patent to Moon et al U.S. Pat. No. 3,585,901 which is directed to a hydraulic pump, provides for noise reduction at a valve plate 52 and cylinder block 14 port plate 51 interface (FIG. 1). The presence of noise is frequently associated with erosion and cavitation wear problems. Moon et al accomplishes noise reduction by the provision of "fishtails" 76, 86 adjacent the leading edge of high and low pressure ports in the valve plate. The fishtails provide orifices that control the rate of bulk modulus flow. This results in a reduction of wave fronts and a decrease in noise and concomittant wear. Moon et al includes a slipper 34 that has a passage through the slipper 34, not referenced, to provide lubrication to the slipper 34 and swashplate bearing surface interface 32. Moon does not recognize the problem of erosion at the slipper-swashplate interface and entertains no remedy.
The Schauer U.S. Pat. No. 4,096,786, directed to a rotary fluid energy translating device, is in the same class of devices as the patents to Slimm and Moon et al in that noise level reduction is a primary feature of the invention and contemplates the inclusion of structure to reduce the noise level of the device during operation by pressure control within the device during transition between high and low pressure ports of the device and, particularly, by means of employing trapped volumes of fluid to obtain intermediate pressure levels during the transition and by varying the trapped volumes for a controlled rate of pressure change dependent upon the volume of fluid in the device subject to pressure transitions. The Schauer patent neither recognizes the slipper erosion problem nor provides any means that would inherently treat the problem.
The patent to Sperry U.S. Pat. No. 1,714,145, directed to a crankless engine, teaches the use of radial grooves 20 in a slipper ring 15 to provide a lubricant flow path to reduce friction between a slipper ring 15 and swashplate 11 (FIG. 2). Sperry does not provide a lubrication path through the pistons 46 as will be described hereinafter with respect to the subject invention.
The patent to Alexander U.S. Pat. No. 3,996,806, involves a hydrostatic transmission with oscillating output which shows in FIG. 1, a rocker arm 30, that carries a plurality of pistons 40. Each piston is provided with a slipper shoe 54 riding on the surface of a cam plate 14. The cam plate is provided with a plurality of passages 56 that communicate with a balance pad 32 on one side of the cam plate 14 as well as the slipper shoe side. The ball 44 and spring 46 act as a check valve and are involved in the pumping of lubricant to balance pad 32. The pressure on either side of the cam plate 14, i.e. at the slipper interface with cam plate and balance pad with the back of the cam plate, "equalize the load on the front and back of the cam plate" (column 2 lines 3 to 5). The Alexander patent does not feature venting of a slipper to reduce cavitation and erosion wear as taught in the specification that follows.